mbed_error: remove redundant memset
mbed_error: remove redundant memset

If we want zero-fill, strncpy does it anyway.

andcor02/mbed-os,kjbracey-arm/mbed,andcor02/mbed-os,mbedmicro/mbed,mbedmicro/mbed,kjbracey-arm/mbed,mbedmicro/mbed,andcor02/mbed-os,andcor02/mbed-os,andcor02/mbed-os,mbedmicro/mbed,mbedmicro/mbed,kjbracey-arm/mbed,kjbracey-arm/mbed,andcor02/mbed-os